1. Field of the Invention
The present invention relates generally to an improved data access arrangement interfacing a modem with a telephone line, and specifically to a data access arrangement which is programmable to perform according to the different specifications in different countries.
2. Description of the Related Art
In order to transmit digital data between computers over the public switched telephone network (PSTN), modems are used to convert between a computer's digital signals and analog signals that can be carried on the telephone network's analog transmission lines. The subscriber portion of the telephone network (PSTN) has two wires known as "tip" and "ring," which carry information being transferred to and from the subscribers, as well as control signals, such as a ring signal. The modem receives a serial stream of bits as input from a computer and produces a modulated carrier as output, thus converting the digital signals of the computer to analog signals for transmission on the telephone wires. Because modems connect to these conventional telephone wires, they must conform to the requirements of the telephone network (PSTN). A data access arrangement (DAA) is connected as an interface between the modem and the telephone network (PSTN) to ensure the modem is compatible with the telephone network (PSTN).
Telephone companies require that a DAA be placed in series between the telephone wires and all equipment, such as modems, connected to the telephone network (PSTN) in order to isolate the equipment from the telephone network (PSTN). The DAA provides this electrical isolation between modems and the telephone network (PSTN), as well as providing impedance matching, hybrid circuit and sometimes amplification, filtering and control functions. DAAs commonly utilize transformers or optical couplers as safety components to provide such electrical isolation.
The telecommunication interface requirements of the DAA circuit are not governed by one international body. As a consequence, countries have differing requirements that the DAA must meet. Each country uses different specifications and parameter values for its telephone network communications. For example, different countries have different requirements for parameters such as Off-Hook DC loop characteristics and Ring Detection requirements. In pulse dialing, different countries have different values for such parameters as make/break ratio, interdigit gap, pulses per second, and make current value. Each country requires equipment to meets its own specifications and parameter requirements in order to connect to the telephone network of that country. Therefore, a user must have a separate modem tailored for each country in which the user wishes to connect to their telephone network.
It is desirable to produce a DAA which could be used in all of the different countries. One conventional solution to this problem is to use a DAA circuit which is external to the modem. The DAA circuit could then be interchanged with a DAA circuit meeting the requirements of the country in which modem is being used. For instance, the DAA assembly could be formed as an integral part of a cable connecting the modem to the telephone line. Different cables, each including a different DAA circuit, could then be interchangeably used to meet the differing international requirements. However, this arrangement then requires several cables to be carried along with the modem, so that the cable possessing the particular international requirements needed at the time can be connected to the modem. In an effort to overcome the hassle of carrying multiple cables with varying DAA parameters, U.S. Pat. No. 5,457,601 issued to Georgopulos et al. discloses a modem configured to receive an interchangeable data access arrangement module. This configuration allows different data access arrangement modules to be interchangeably connected to the modem to meet the varying country-specific interface specifications. While the data access arrangement modules are less bulky than external cables, this arrangement similarly has the disadvantage of requiring multiple modules having different data access arrangement specifications be transported along with the modem in order to change the operating characteristics of the data access arrangement.
There have been attempts to provide DAAs which can be programmed to different country specifications using the firmware of the DAAs, so that the particular DAA circuits are not required to be interchanged to accommodate the particular country specifications needed. One such attempt is disclosed in U.S. Pat. No. 5,712,977 issued to Glad et al. which teaches using a universal modem that can be reconfigured to meet different country specifications. The universal modem is configured according to a country identification code input by a user, where the modem includes a memory having a stored relationship between the country identification code and the DAA specifications for that country. The modem includes a microprocessor which retrieves the DAA specifications associated with the country code input by the user and programs the DAA with control data according to those specifications. The microprocessor sends the control data to the DAA as a serial bit stream which is interpreted by the DAA to decipher the control data. This type of DAA, which is formed as a programmable IC, requires intelligent components to process the control data in the serial bit stream. DAAs of this type are active circuits which further require a power supply to drive the programmable DAA. Thus, programming a DAA using a serial bit stream requires an intelligent, programmable IC as well as an active circuit to drive the programmable IC, which adds to the cost and complexity of the DAA.
There is a need for a DAA which can be configured to various country specifications using a passive configuration (i.e., having components which do not require a power source) and which also does not require intelligent components to interpret commands coming from the modem in a serial stream. In order to accomplish these needs, there have been attempts to use dedicated components to produce command signals from the modem to the DAA informing the DAA to operate according to particular parameters. Each dedicated component emits a command signal corresponding to a respective country specification. Such commands from dedicated components are delivered to the DAA is parallel form, so that the DAA does not require intelligent components to interpret the contents of the incoming command. Further, the power for delivering the commands comes from the dedicated components themselves, and the DAA can be formed using a passive circuit configuration.
As discussed above, each country has different requirements for parameters such as Pulse Dialing, Off-Hook D.C. loop characteristics, and ring detection requirements. For example, there are currently two different pulse dialing parameters and four different Off-Hook D.C. loop parameters used by most of the countries in the world. Each DAA includes pulse dialing circuitry operating according to the required pulse dialing parameters for that country as well as D.C. loop circuitry operating according to the necessary D.C. loop characteristics for that country. When using dedicated components to program a DAA to function according to one of the possible operating parameters typically used worldwide, six dedicated components would be required in the above example to deliver two different possible pulse dialing parameters to the pulse dial circuitry and four different possible Off-Hook D.C. loop parameters to the D.C. loop circuitry of the DAA. Since the dedicated components are located in the modem, a safety barrier, such as a transformer or optical coupler, must be respectively positioned in series between each circuit and the modem to provide electrical isolation for all electrical pathways between the telephone network (PSTN) and the modem. Thus, a DAA capable of being programmed for the two different pulse dialing parameters and four different Off-Hook D.C. loop parameters used by most of the countries in the world would require six safety barriers, one for each pulse dialing parameter and D.C. loop parameter. In manufacturing DAAs, approximately 50% of the cost of producing a DAA comes from the cost of the safety barriers themselves. The cost of producing a DAA can be reduced dramatically by reducing the number of safety barriers required by the DAA.
There is a need for a programmable DAA having a passive circuit configuration which reduces by the number of safety barriers required for providing electrical isolation between a modem and the telephone network (PSTN) for signals transmitted from the modem to program the DAA with various possible operating parameters in order to significantly reduce the cost, complexity, and size of the DAA.